<!DOCTYPE html>
<html>
<body>
<canvas id="c" width="500" height="200"></canvas>
<div style="position:absolute; left:0; top:0; height:100px; width:100%; background:yellow"></div>
<script>
var c = document.getElementById("c");
var ctx = c.getContext("2d");
ctx.fillStyle = "blue";
ctx.fillRect(0, 0, c.width, c.height);
</script>
</body>
</html>
